In this supplementary material we demonstrate the accuracy of our ab initio simulations and validate our slab model of the H-C(100) diamond surface. All calculations are performed using version 5.4.4 of the Vienna Ab initio Simulation Package (VASP) using the projector augmented wave method[1]}, [2]}, [3]}, [4]} with PBE functionals[5]}. In Section  we determine the plane wave cut-off energy required to adequately simulate bulk diamond and optimize the lattice constant for all further calculations. In Section  we investigate the relationship between slab thickness, the H-C(100) work function, and the energetics of C insertion into the C–N dimer. In Section  we investigate the relationship between the lateral dimensions of the slab, intra-cellular interactions between N defects, and the energetics of C insertion into the C–N dimer. Using these results we identify the slab dimensions required to adequately represent the physics and chemistry of the diamond surface. Finally, in Section  we further investigate the instability of state 5 during C insertion into the C–C dimer.
